**4. Fan mechanism as a source of dynamic events in the earth's crust**

#### **4.1 Features of the fan-structure formation in complex natural faults**

This section discusses the role of the fan mechanism in generation of shallow earthquakes and shear rupture rockbursts in deep mines. In the previous sections, we introduced different unique features of the fan mechanism and 'abnormal' properties of hard rocks. All analysis was conducted for primary shear ruptures which are thin and continuous. Unlike primary ruptures natural faults typically have very complicated segmented and multi-hierarchical structure [7, 33, 34]. Main principles of the complex fault evolution in association with the fan mechanism were discussed in [20, 23, 35]. Here we will outline briefly most important features of the fan-mechanism generation in complex faults.

It was observed that in ultra-deep South African mines, very severe dynamic events (shear rupture rockbursts) are caused by new shear ruptures generated in pristine rock [36, 37]. These mine tremors are seismically indistinguishable from natural earthquakes and share the apparent paradox of failure under low shear stress [37]. Photographs of such faults are shown in **Figure 5b**. The structure of all these faults is identical consisting of a row of domino blocks. However, the domino structure is more complex than in primary ruptures.

**Figure 13** explains features of this structure formation. Series of photographs in **Figure 13a** (modified from [38]) shows principles of segmented fault propagation observed experimentally. The fault propagates due to advanced triggering of new segments. The photographs show four stages (I–IV) of the fault evolution. Segments are represented here by white lines. The fault propagates from left to right. The segments are generated one by one due to the stress transfer and propagate bilaterally. At the meeting of each two neighbouring segments, they are connected by a compressive jog. It was found out in [29, 38] that jogs of the compression type are very common at high confining pressures to fault zones regardless of their sizes. Overlap zones of the jogs

*Earth Crust*

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**Figure 13.**

**Figure 12.**

*a) Shear rupture propagation by advanced triggering of new segments (modified photograph from [29]). b)* 

*(a) Variation of the fan-mechanism efficiency ψ = τf/τfan versus confining pressure σ3 for rocks of different hardness (UCS). (b) Variation of the optimal efficiency of the fan mechanism ψopt versus UCS.*

*and c) Principle of formation of the domino and fan structure in segmented faults.*

**Figure 14.**

*The fan mechanism is predominantly activated in fault segments of lower hierarchical ranks (schematic illustration) providing extreme dynamics along thin localised zones.*

are subjected to significant irreversible deformation. **Figure 13b** and **c** demonstrates that in brittle rocks the irreversible deformation in jogs is associated with formation of a row of domino blocks (modified photograph from [29]). The general fault here consists of a number of segments represented by primary ruptures. The propagation of primary ruptures in hard rocks at high σ3 is governed by the fan mechanism.

The domino structure of the next hierarchical ranks can also be involved in the fanstructure formation. This feature is illustrated in **Figure 14**. It was observed in [38] that segmentation as a mechanism of fault propagation acts on all hierarchical ranks of complex faults. Once a number of segments of a given hierarchical rank coalesce, they behave as a whole as a new and longer segment of one higher rank. Segment of higher rank can trigger a new segment (shear fracture) at greater distance. A photograph in **Figure 14a** (modified from [29]) shows a fault fragment involving segments of three hierarchical ranks. The structure of this fault is shown symbolically on the left. It incorporates primary ruptures and higher rank segments formed on the basis of compressive jogs represented by the domino structure (rank II and rank III). Domino blocks involved in segments of higher rank can form the fan structure similar to primary ruptures due to rotation of them caused by shear displacement of the rupture faces. However, the complete fan structure can be formed if shear displacement between the fault faces dfault is sufficient for the completed block rotation.

**Figure 14b** shows the initial and final positions of domino blocks for two shear ruptures of thicknesses h1 and h2. The thick rupture requires significantly

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**Figure 15.**

*low field shear stresses caused by the fan mechanism.*

*Dramatic Weakening and Embrittlement of Intact Hard Rocks in the Earth's Crust at Seismic…*

greater displacement dfault = Δ to complete the block rotation. Due to this, the complete fan structure (red zones in **Figure 14a**) is predominantly created in segments of lower ranks. The fan mechanism generated here creates high dynamics of the failure process. Relatively thin localised zones of very intense destruction can be observed in each dynamic fault. The initial domino structure of these segments is completely destroyed by extensive and violent shear and represented by pulverised gouge. In high-rank segments, domino blocks rotate by low angles without formation of the fan structure and assist the accommodation of displacement along the whole fault. The domino-like structure is typical for faults of very different scales including laboratory specimens, shear rupture rockbursts in mines and earthquakes. Two photographs in **Figure 14a** and **c** show identical domino structure of two dynamic faults which generated severe shear rupture rockburst (a South African ultra-deep mine) and earthquake (the San Andreas

**4.2 Generation of new faults in intact rock at low shear stresses nearby a pre-**

This section proposes an alternative explanation to the fact that earthquakes are commonly attributed to pre-existing faults. Pre-existing discontinuities play the role of local stress concentrators, creating the starting conditions for the fanstructure formation. After completion of the initial fan structure, it can create a new dynamic fault in the form of earthquake by propagation through intact rock mass loaded by low shear stresses. **Figure 15** illustrates one of the many models for generation of high local stress on the basis of pre-existing fault. It shows a rock fragment involving a pre-existing fault (black line) with a compressive jog. This

*Features of generation of a new extreme rupture in pristine hard rock in the vicinity of a pre-existing fault at* 

*DOI: http://dx.doi.org/10.5772/intechopen.85413*

fault exposed on land) [39].

**existing fault caused by the fan mechanism**

*Dramatic Weakening and Embrittlement of Intact Hard Rocks in the Earth's Crust at Seismic… DOI: http://dx.doi.org/10.5772/intechopen.85413*

greater displacement dfault = Δ to complete the block rotation. Due to this, the complete fan structure (red zones in **Figure 14a**) is predominantly created in segments of lower ranks. The fan mechanism generated here creates high dynamics of the failure process. Relatively thin localised zones of very intense destruction can be observed in each dynamic fault. The initial domino structure of these segments is completely destroyed by extensive and violent shear and represented by pulverised gouge. In high-rank segments, domino blocks rotate by low angles without formation of the fan structure and assist the accommodation of displacement along the whole fault. The domino-like structure is typical for faults of very different scales including laboratory specimens, shear rupture rockbursts in mines and earthquakes. Two photographs in **Figure 14a** and **c** show identical domino structure of two dynamic faults which generated severe shear rupture rockburst (a South African ultra-deep mine) and earthquake (the San Andreas fault exposed on land) [39].
